1. Field of the Invention
The present invention relates generally to rate responsive cardiac pacemakers, and more particularly to an implantable microprocessor-controlled rate-responsive pacemaker wherein a programmed rate or a sensor-indicated rate may be selectively used to determine the rate at which pulses are generated on demand, or as otherwise programmed, by the pacemaker; one feature of the invention is an automatically adjustable rate response threshold which a sensed physiological parameter must exceed before a rate-responsive function is provided.
A pacemaker is an implantable medical device which delivers electrical stimulation pulses to a patient's heart in order to keep the heart beating at a desired rate. Early pacemakers provided stimulation pulses at a fixed rate or frequency, such as 70 pulses per minute (ppm), thereby maintaining the heart beat at that fixed rate. Subsequently, pacemakers were designed to not only stimulate the heart, but also to monitor the heart. If a natural heart beat was detected within a prescribed time period (usually referred to as the "escape interval"), no stimulation pulse was delivered, thereby allowing the heart to beat on its own without consuming the limited power of the pacemaker. Such pacemakers are referred to as "demand pacemakers" because stimulation pulses are provided only as demanded by the heart.
Early demand pacemakers had a fixed base rate associated with them. In later versions, the base rate was programmably selectable, and thereafter became commonly known as the "programmed rate." If the heart was able to beat on its own at a rate exceeding the base (or programmed) rate, then no stimulation pulses were provided. However, if the heart was not able to beat on its own at a rate exceeding the base rate, then stimulation pulses were provided to ensure that the heart would always beat at least at the base (or programmed) rate. Such operation was achieved by simply monitoring the heart for a natural beat during the escape interval. If natural activity was sensed, the timer which defined the escape interval was reset. If no natural activity was sensed, a stimulation pulse was provided as soon as the escape interval had timed out. Changing the base (or programmed) rate was accomplished by simply changing the duration of the escape interval.
In recent years, rate-responsive pacemakers have been developed which automatically change the rate at which the pacemaker provides stimulation pulses as a function of a sensed physiological parameter. The physiological parameter provides some indication of whether the heart should beat faster or slower, depending upon the physiological needs of the pacemaker user. Thus, for example, if a patient is at rest, there is generally no need for a faster-than-normal heart rate, so the rate-responsive pacemaker maintains the "base rate" at a normal value, such as 60 pulses per minute (ppm).
However, if the patient is exercising, or otherwise physiologically active, there is a need for the heart to beat much faster, such as, for example, 100 beats per minute. For some patients, the heart is not able to beat faster on its own, so the pacemaker must assist. In order to do this effectively, the physiological need for the heart to beat faster must first be sensed, and the "base rate" of the rate-responsive pacer must be adjusted accordingly. Hence, rate-responsive pacemakers are known in the art which increase and decrease the "base rate" as a function of sensed physiological need.
Numerous types of sensors are taught in the art for use with a rate-responsive pacer. One common type of sensor is an activity sensor which senses the physical activity level of the patient. See, for example, U.S. Pat. Nos. 4,140,132, to Dahl, and 4,485,813, to Anderson et al. In accordance with the teachings of Dahl or Anderson et al., a piezo-electric crystal is used as an activity sensor. Such a crystal generates an electrical signal when subjected to physical movement and stress according to well known principles.
The electrical signal generated by the crystal may be processed by rectifying and filtering it as taught by Dahl, or by monitoring the frequency of the highest amplitude peaks as taught by Anderson et al. An increase or decrease in the parameter being monitored signals a need to increase or decrease the rate at which pacing pulses are provided. Note, as used herein, the term "pacing rate" refers to the rate at which the pacer provides stimulation pulses, or in the case of demand pacers, the rate at which the pacer would provide stimulation pulses in the absence of naturally occurring heart beats. Also, for purposes of this application, the terms "pacer" and "pacemaker" are used interchangeably.
Other types of sensors used in prior art rate-responsive pacers include sensors that sense respiration rate, blood oxygen level, blood and/or body temperature, blood pressure, the length of the Q-T interval, the length of the P-R interval, etc. Rate-responsive pacers using these other types of sensors have yet to demonstrate their commercial viability. To applicants' knowledge, only the piezoelectric sensor has been marketed successfully in significant numbers to date. However, any or all of these other types of sensors may prove efficacious in the future. Advantageously, the invention presented herein may be used with any of these prior art sensors, or with any other physiological sensors yet to be developed.
Even when a reliable or quasi-reliable indicator of physiological need is used in a rate-responsive pacer, however, there is still a need to customize the manner in which a particular patient reacts to the output signals from the chosen sensor. While some flexibility exists in this regard in the manner in which a pacemaker is programmed, the available programming options relative to the rate-responsive features have heretofore been severely limited. Further, even when a pacemaker is initially programmed in a suitable manner, there is no guarantee that this manner of programming will remain suitable over a long period of time.
In addition, while reprogramming may typically be performed, such reprogramming requires additional visits to the doctor, which visits can become quite burdensome for the patient. Hence, there is a need in the art for a rate-responsive pacer that provides greater flexibility in the manner in which the pacer is initially programmed, and which thereafter provides automatic adjustment of some of the key parameters which influence the effectiveness of the pacer.
Moreover, because all rate-responsive pacers include some type of sensor or sensing mechanism to sense at what rate the heart should be paced, there is a critical need to ensure that the sensor, and its related circuitry, function properly. Should the sensor fail, or should any of the circuits associated with the sensor fail, the pacer must still continue to provide stimulation pulses, if required, at a safe rate. In this regard, it is noted that normally, because of the stringent design and manufacturing requirements imposed on an implantable medical product, failure of the pacemaker or the pacemaker circuits and elements is an extremely unlikely event. However, because the sensors used with a rate-responsive pacer involve additional parts, and because the operation of such sensors typically involves measuring or sensing very ill-defined and/or low level signals, and further because the processing circuitry used with such sensors is by necessity quite sophisticated and complex in order to extract the relevant information from the low level signals, the possibility of a circuit failure in a rate-responsive pacer is increased. Hence, what is needed in the art is a fail-safe mechanism within the rate-responsive pacer which may be used to provide stimulation pulses at a safe rate in the event of failure of the sensor and/or rate-responsive portions of the pacemaker circuits.
Similarly, because of the above-mentioned complexity and sophistication of most rate-responsive pacers, such pacers are typically expensive to design, manufacture, test, and maintain. It is not uncommon, for example, for a particular design approach to be taken relative to a proposed rate-responsive pacer design, only to discover several months or years later that the approach is not the best approach that should or could be taken. Further, because of the miniaturization of the circuits used in modern implantable pacers, any significant design change usually requires starting over with the design of a new custom integrated circuit chip. Such a process of starting over once a new approach is taken is not only time consuming, but is also extremely expensive.
What is needed, therefore, is a rate-responsive pacer configuration which allows the use of a tried and proven pacemaker chip (pulse generator and related controls) for the basic pacemaker functions, yet allows a versatile, easy-to-change processing circuit chip, used with a selected sensor, for the rate-responsive functions. This is a primary objective of the present invention. Finally, the present invention must attain all of the aforesaid advantages and objectives without incurring any substantial relative disadvantage.